The potentially high photoluminescent quantum yield in organic semiconductors has made light emission through charge injection under an applied field (electroluminescence) a reality. This has thus resulted in the use of organic materials for LEDs, with the use of polymers having an advantage over smaller organic compounds in improving structural stability and processibility of the organic layers.
The first polymer-based LEDs consisted of an emissive layer of poly(1,4-phenylene vinylene) (PPV) sandwiched between indium tin oxide (ITO) as the hole injection electrode and aluminium or calcium as the electron-injecting electrode (Burroughes et al., Nature, 1990, 347, 539). This has been followed in other polymers for LED applications, with emphasis on the tuning of the emission colour by controlling the substitution pattern and the effective conjugation length in the polymer.
The route used for the synthesis of PPVs for LEDs has been predominantly the Wessling sulphonium salt precursor route (U.S. Pat. No. 3,401,152/1968 Wessling et al.) while another important method of Gilch (Gilch et al., J. Poly. Sci. 1-A, 1966, 4, 1337), involved dehydrohalogenation of bis(halomethyl) benzene monomers with excess potassium t-butoxide to directly obtain the conjugated polymer. This protocol has been applied to the red emissive poly(2,5-dialkoxy-1,4-phenylene vinylene)s which have a typical absolute PL efficiency of ca. 15-20%. Most commonly the preparation has referred to poly[2-methoxy-5(2′-ethylhexyloxy)-1,4-phenylene vinylene] (Wudl, U.S. Pat. No. 5,189,136/1990). A modification of this route by Swatos and Gordon, based on the chloro-precursor route, used one equivalent of base to form the chloro-substituted soluble precursor polymer, which was then thermally converted to form the fully conjugated polymer [Swatos and Gordon, Polym. Prepr., 1990, 31 (1), 505]. This route has since been used by various groups to make alkyl-, aryl-, and alkoxy-substituted PPVs (Hsieh et al., Adv. Mater., 1995, 7, 36; Polym. Adv. Tech, 1997, 8, 392; U.S. Pat. No. 5,558,904; Sarnecki et al., Synth. Met., 1995, 69, 545).
There is still a need to provide photoluminescent and electroluminescent polymers, which are easily manufactured and are processible. There is also still a need for polymers having improved photoluminescence and electroluminescence efficiencies.